1. Field of the Invention
The present invention relates to electronic countermeasures and, more specifically, to devices used to detect harmful electromagnetic signals.
2. Description of the Prior Art
The United States has been a tempting terrorist target for decades. With the successful attack on the New York Trade Center in February 1993 there has been speculation by terrorism experts that attacks against the U.S. may increase. At the same time, a weapon is now available to subversive organizations against which there are few defenses. In fact, apart from the catastrophe itself, the weapon will leave no evidence of the attack.
Advanced electronic systems, such as aircraft control systems, are increasingly vulnerable to interference by electromagnetic radiation. Radio frequency (RF) power has been shown in many Department of Defense (DoD) and Department Of Energy (DOE) studies to be a threat to the proper operation of modem electronic systems. As circuits have become more densely packaged, more energy efficient and operate at higher speeds, they've experienced an associated increase in vulnerability and susceptibility to perturbations from non ionizing radiation.
This inherent vulnerability to RF power has spawned research internationally in optimizing transmitters for use as weapons. RF weapons have been developed by NATO and former Soviet nations. The effectiveness of many of these weapons is documented in studies sponsored by DoD and DOE organizations. Anecdotal data suggest RF weapons may have been deployed successfully in combat.
The effects of a successful RF weapon attack are unpredictable. The goal of RF weapons is merely to disrupt their victim system, with the specific result being of secondary importance. The end result varies widely depending on the victim system. In some cases, computers may be reset. In other cases, local oscillators may be driven off frequency, navigation systems misguided, safety sensors incorrectly set or reset, faulty data recorded or control systems given erroneous inputs. In the case of airliners, intended victim subsystems provide control of elevators, thrust reversals, etc.
The significance of the perturbation is proportional to the importance of the system corrupted. A portable compact disc player may react by garbling music or changing the track it is playing. Were a similar amount of energy directed at a commercial aircraft, the plane's control and navigation systems could be corrupted enough to cause a crash.
Another important similarity shared by vulnerable systems involves post-attack evidence. There typically is none. Although the perturbation of the victim system is indisputable while subjected to the RF field, the affected circuits rarely show damage. This makes identification of the cause extremely difficult. Failures appear to be anomalies with no traceable cause. One aspect of a successful RF weapon attack is that no evidence remains to incriminate the perpetrator.
The effects of even low level electric fields have long been recognized by the civil aviation community. Regulations prohibiting airline passengers from using radio receivers in flight are in response to the threat such systems pose to the plane's electronics systems. Superheterodyne receivers, such as those in commercial AM/FM radios, contain small local oscillators, which generate RF energy. Signals generated by the local oscillators are small. Signals escaping the radio as RF energy are unintentional and therefore likely to be smaller still. Even these small RF signals threaten the stability and reliability of the aircraft's electronics.
As aircraft become more advanced, their vulnerability increases. As the electronics become more integral with the actual control of the plane, the results can be more catastrophic. Boeing claims that the Boeing 757 can take off, fly to its destination and land itself without pilot intervention. Indeed, the plethora of gauges formerly found in the cockpit has been mostly replaced by a single screen. On-board computers decide what information the pilot needs to see displayed on the screen. Many of the moment to moment decisions made in flying a plane have been shifted to built-in electronics. Disrupting these systems can cause a plane to execute erroneous commands resulting in a crash.
As aircraft electronics systems are becoming more sensitive, the population of potential sources of interference is exploding. Portable Electronic Devices PEDs) commonly used in flight include laptop computers, hand held games, telephones, personal organizers, compact disc players, etc. Many of these PEDs emit RF energy. Instances of PEDs apparent interference with on board electronics systems have become increasingly common.
Similar to an RF weapon attack, no evidence of the disruption remains for analysis after the interfering signal stops. This has made locating failures difficult. Cockpit personnel have resorted to going through the cabin and asking individual passengers to turn off their PED. Correlating the aircraft's return to normal operation with which PED was disabled simultaneously has been the most powerful tool in identifying the source of the problem.
The population of cellular phone users is also exploding. Cellular phones are RF transmitters, and are prohibited from use in flight. According to Aviation Week & Space Technology, (Mar. 8, 1993 p 33): "Narratives from the incidents involving cellular phones show that the pilots noticed a clear cause and effect relationship between cellular/navigation interference and a cellular phone being illegally used in the cabin."
RF weapons are an ideal tool for terrorists. They present a combination of: (1) a proven vulnerability in one of terrorists' favorite targets--airliners; (2) the fact that ground based systems can allow a non-suicidal attack; and (3) the fact that the cause for the attack cannot be identified.
Announced restrictions of PEDs during approach and takeoff indicate and advertise when the aircraft is at greatest risk. Coincidentally, it also represents the terrorist's greatest opportunity. If on-board systems have been disrupted by PEDs inadvertently emitting fractions of watts, an attack focusing billions of watts on the plane may also disrupt the systems. During approach and takeoff, vulnerability is increased by low airspeed and altitude. Pilots have less time to react to system failures and few options to exercise. During this critical phase, the planes are over uncontrolled ground and at close range. This provides the terrorist access.
The lack of residual evidence is critically important to terrorists. It increases the probability of success for subsequent attacks, since potential targets will not be able to identify the risk against which they must protect themselves. At the same time, it decreases the probability of apprehension during any attack.
There are a number of weapons of mass destruction that terrorists would like to use, but are controlled by the sane powers in the world. This is generally done by tightly limiting access to critical technology or components. In some cases, the weapon system is too expensive or visible for a subversive organization to procure.
None of these safeguards are applicable to RF weapons. The technology was explored on both sides of the iron curtain under strict security for some time. In the past few years, however, the technology has moved into the physics labs of universities in the West. This provides a subversive organization with the opportunity to learn how to build such a system from scratch, using no controlled components or subsystems.
More recently, an even greater opportunity emerged. The former Soviet Union is selling most of its technology on the open market. In June 1993 a large sale in London reportedly featured many technologies that had been closely guarded secrets. A subversive may no longer need to learn how to build an RF weapon. Already tested RF weapons may be available for use. Scientists who developed the Soviet systems are also available, having lost their jobs with the collapse of their government. These experts represent a pool of highly skilled operators for RF weapons that have been acquired through the open market or left on battlefields.
On Mar. 3, 1991, United Airlines flight 585 crashed on approach to Colorado Springs. The National Transportation Safety Board (NTSB) has not been able to determine a cause for the crash. Their studies show the controls responding erratically and they are unable to reproduce the conditions of the crash in a simulator. Pilot autopsies and cockpit voice recordings indicate that the flight crew was competent and actively responding to the malfunctions. These are the exact indications an RF weapon attack might cause and the same residual evidence in the aftermath.
While UAL 585 may have crashed for reasons other than an RF weapon attack, it serves as an illustration of how such an attack would look. The flight crew had complete loss of control of the plane. Something besides pilot input caused the plane to bank sharply and crash in an almost directly nose-down attitude. Data show the plane pulled 4 g's in turning and was actually headed slightly in the opposite direction from its intended path when it crashed.
Detection of RF weapon emissions is difficult. The weapon is designed to produce the maximum amount of power possible, and to concentrate the power into the shortest possible pulse. Without information of when a pulse will occur or what the frequency it will be, most receivers cannot detect the use of such a weapon. Indeed, the weapon's probable effectiveness is inversely proportional to its probability of being detected by most receivers.
An effective detector must respond to short pulses individually. Pulses are likely to be much less than a microsecond in duration. Although the pulse width is likely to be short, the time between pulses can be expected to be long. Furthermore, one pulse may differ significantly from the next pulse. Each pulse must be considered a separate event for detection.
Defense studies have shown large scale "hardening" of electronic circuits against RF interference is difficult. Furthermore, a victim system's vulnerability is highly frequency dependent. Individual design and manufacturing details of the victim system inadvertently "tune" it to be sensitive to particular frequencies. Disrupting the victim system depends on locating and exploiting these vulnerable frequencies.
Generally, frequencies of vulnerability must be found experimentally. For the RF terrorist, this means a trial and error process of picking a frequency, attacking a plane and watching for an effect. During this experimentation phase, the terrorist has a low probability of success. However, without sensors deployed to detect his trials, he has even less probability of being discovered.
Once a frequency of vulnerability is identified, the probability of success for subsequent attacks increases dramatically. A goal of modern design and manufacturing processes is uniformity. Because a given model of aircraft has common design parameters, every aircraft of a given model has certain common vulnerabilities. Thus, before finding frequencies of vulnerability, the probability of success is relatively low. A quantum change in that probability occurs with the identification of the first such frequency. The terrorist's probability of apprehension is unaffected by the identification of such frequencies, however. The risk of being caught is solely dependent on the fielding of equipment that can detect his emissions.
The effects of on-board transmitters, such as cellular telephones, can have the same effect if the plane has systems sensitive to the transmitted frequency. The risk is mitigated somewhat by the relatively few frequencies that must be accounted for. It is amplified, however, by the size of the population owning cellular telephones and being tempted to use them during the most dangerous part of the flight, final approach. Making a "quick" phone call ahead to confirm hotels and ground transportation may seem innocent, but it has the potential to cause the flight crew to loose control at a critical part of the flight.
No device exists which generates an alarm upon receipt of a radio-frequency signal having an amplitude above a predetermined threshold and not matching any known signals of high amplitude.